Hydraulic pump and piston for such a hydraulic pump

ABSTRACT

The invention relates to a hydraulic pump, in particular an adjustable axial piston pump, having at least one piston ( 22 ) which is movable in a reciprocating manner in a longitudinal direction within a pump housing during operation of the hydraulic pump, said piston ( 22 ) having a link head ( 24 ) and a piston top ( 54 ) opposite the link head ( 24 ), and also at least one hollow chamber ( 60 ) which is surrounded at least partially by a piston housing ( 62 ) which substantially or completely terminates each hollow chamber ( 60 ) towards the outside. The invention also relates to a piston ( 22 ) for such a hydraulic pump.

The invention relates to a hydraulic pump, in particular an adjustable axial piston pump, as well as a piston for such a hydraulic pump.

In the operation of such a hydraulic pump, mechanical energy is converted to hydrostatic energy, wherein the mechanical energy is supplied, for example, by an electric motor. In the structure of an axial piston pump, hydraulic pumps of this type normally have numerous cylinders, which are attached to a rotatable shaft such that they extend in the axial direction, and are disposed on a partial circle, offset at a radius to the shaft axis. Pistons are displaceably disposed in the cylinders. The pistons, which rotate with the cylinder, are moved, for example, by an adjustable inclined plate. For this, in order to minimize the friction, a piston/sliding shoe connection is provided between the inclined plate and the pistons.

One problem thereby is the mass of the rotating piston, which is normally made of a single piece. Because of this mass, significant energy is needed to accelerate the piston, and to subsequently decelerate it, this being associated with significant wear in the region of the piston/sliding shoe connection. In order to reduce the inertia of the mass, a bore hole has been provided previously, in the base of the piston, such that the piston is substantially hollow. A bore hole of this type in the piston has the disadvantage, however, that the dead volume in the cylinder is increased. As a result of the dead volume, there are compression losses and increased volume flow, or pressure, fluctuations, which are a reflection of a lower pumping efficiency.

Based on this background, the object of the invention is to present a hydraulic pump and a piston for such a hydraulic pump, with which, or by means of which, respectively, the efficiency of the hydraulic pump is or will be increased.

The first part of the object is achieved by means of a hydraulic pump having the features of claim 1. Advantageous embodiments and further developments of the hydraulic pump can be derived from the dependent claims 2 to 10. The second part of the object is achieved by means of a piston having the features of claim 11. Advantageous embodiments of the piston are the subject matter of claims 12 and 13.

According to the invention, the pump, in particular in the form of an adjustable axial piston pump, has at least one piston that can move back and forth in a longitudinal direction inside a pump housing during operation of the hydraulic pump. The piston has a link head and a piston base lying opposite the link head, as well as at least one hollow chamber. The hollow chamber is encompassed, at least partially, by a piston housing. The piston housing closes the respective hollow chamber, substantially or entirely, to the outside.

To the extent that reference is made, presently and below, to the hollow chamber being substantially closed off to the outside, this means that, starting from the environment, no relevant media discharge, in particular in the form of a fluid, occurs through any existing openings in the piston housing and/or in its end piece, taking the form of a lid, for example, into the interior of the respective hollow chamber. An effective barricade of this type can still be achieved when the hollow chamber is substantially closed, even when, for example, openings are to be formed in the piston housing, such as capillary holes. Substantially closed thus means that even when such openings are formed, passing through the hollow chamber, these nevertheless prevent, in a damping manner, or prohibit, in the manner of a capillary hole, accordingly, the flow through, in the manner of a choke or a screen.

The hydraulic pump according to the invention is distinguished in that the advantage of having a piston with a lower mass is not obtained at the cost of having a larger dead volume as a result of a hollow chamber that is open to the pump housing. Consequently, the inertia forces are advantageously reduced with a minimized dead volume in the cylinder. Consequently, the efficiency of the hydraulic pump is improved, because less energy must be used in order to convey the same quantity of fluid. Furthermore, a hydraulic pump of this type is distinguished by less wear in the region of the piston/sliding shoe connection.

In a particularly advantageous manner, the at least one hollow chamber runs in the direction of the longitudinal axis or parallel to the longitudinal axis of the piston in the piston housing, and passes through the piston, starting from an axial spacing of the link head to the piston base. A bore hole of this type can be readily made in the piston, starting from the piston base. An eccentric configuration of the at least one hollow chamber has the advantage that, in addition to the hollow chamber, a concentric through hole can be provided in the piston, in order to be able to relieve and lubricate the link head during the operation of the hydraulic pump.

Advantageously, the at least one hollow chamber is closed by the piston housing or by a lid that can be attached to the piston housing. With the lid solution in particular, there is the possibility of providing the hollow chamber in the form of a bore hole in the piston, and to subsequently close the hollow chamber with the lid. The lid can likewise be produced as a rotating part, and be placed in the piston housing with no substantial gaps. As a result, fissures are prevented during the subsequent welding, and it is possible to entirely seal the hollow chamber.

The lid and/or the piston can have an equalizing mechanism, which connects the at least one hollow chamber to the environment in a media conducting manner. The development of excess pressure in the hollow chamber can be prevented by means of the equalizing mechanism, which pressure could lead to process instabilities during the joining.

The equalizing mechanism can comprise a spiral groove along an outer circumferential surface of the lid, which is covered when the lid is placed in the housing by the piston housing, except for at a discharge point, wherein the discharge point forms, as part of the equalizing mechanism, the media conducting connection, via the spiral groove, between the environment and the at least one hollow chamber in the piston. It is possible to dissipate an excess pressure during the overall process of inserting the lid in the piston housing by means of the spiral groove.

Particularly advantageously, the spiral groove opens into a bevel in the lid, or borders this bevel, at its other end, the intake point lying opposite the discharge point, wherein the outer diameter of the bevel preferably decreases continuously toward the link head, up to a final diameter. As a result of this bevel, it is possible to ensure that, with numerous hollow chambers, they are all ventilated in a uniform manner during the joining of the lid.

The piston housing can advantageously have a head part at the piston base, by means of which the lid can be coupled to the piston base, preferably by means of welding or brazing. The head part forms a guide and/or attachment possibility for the lid on the piston housing. In this manner, the orientation of the lid is ensured. This is particularly advantageous in the case of a subsequent welding of the lid to the piston housing.

The lid can be permanently connected to the piston housing by brazing or an electron beam welding process, and the piston can preferably be heat treated after the welding. Prior to brazing, the piston housing and the lid are rough-turned, so that the brazing gap between the lid and the piston housing, in particular the walls of the piston housing, is 80 μm to 120 μm, in order to obtain an optimal capillary effect, such that the brazing gap between the piston and the lid fills with solder. The lid should additionally be provided with a sufficiently large bevel on the side facing the bore holes, in order to prevent oxidation during the brazing in a vacuum. After the turning, the lid is inserted in the piston housing, and the two are non-detachably connected to one another in a material bonded manner in a vacuum, using a metallic filler material, a solder, at 450° C. to 1,200° C. This process is also referred to as joint soldering. The hollow chamber formed by the bore holes is closed in this manner. As a result, it is also not possible for the dead volume of the piston to increase during the ongoing operation due to a leak in the piston housing/lid connection.

The electron beam welding seam advantageously closes at least the discharge point of the spiral groove. In this manner, an entry of fluid into the at least one hollow chamber of the piston is impeded significantly.

There are preferably numerous hollow chambers in the piston, preferably having the same diameter, provided on a partial circle, offset to one another. This enables the volume of the hollow chamber to be maximized, and at the same time, a central bore hole running in the direction of the longitudinal axis to be provided, which can be used for lubricating the link head during ongoing operation.

The invention shall be explained in greater detail below based on exemplary embodiments depicted in the figures. Therein:

FIG. 1 shows a detail of a cross section through an axial piston pump;

FIG. 2 shows a cross section of the piston;

FIG. 3 shows a perspective view of the piston with a lid;

FIG. 4 shows a detail of FIG. 2, wherein the production of the electron beam welding seam is shown;

FIG. 5 shows a perspective view of a piston according to the invention, having a lid affixed thereto; and

FIGS. 6 to 8 show a top view, a side view, and a perspective view of the lid.

A detail of a hydraulic pump 10 in the form of an axial piston pump is shown in cross section in FIG. 1. Stationary cylinders 14 are disposed on a rotatable shaft 12 as part of the pump housing 16, such that the cylinders 14 rotate together with the shaft 12. An odd number of cylinders 14 are provided, which are disposed on a partial circle, offset to one another. The longitudinal axes 18 of the cylinders 14 all have the same radius RZ to the longitudinal axis 20 of the shaft 12. A piston 22 that can be displaced longitudinally is disposed in each cylinder 14. A spherical surface 26 is provided on a link head 24 of the piston 22, which is retained in a corresponding receiving part 28 of a sliding shoe 30. The sliding shoe 30 is provided with a longitudinal bore 32 for lubricating the receiving part 28. The sliding shoe 30 is supported on an adjustment mechanism such that it can slide, with an inclined plate 36 disposed therebetween. A fluid, in particular a hydraulic fluid, can be drawn into the cylinder 14 in a first stroke via a fluid inlet 38. The fluid can be subsequently discharged from the cylinder 14 in a second stroke via a fluid outlet 40. Thus, during a half rotation of the shaft 12, a cylinder 14 is filled, and during the other half of the rotation of the shaft 12, it is again emptied. It is possible to obtain a high and uniform pump performance in this manner.

The piston 22 according to the invention is shown in detail in FIGS. 2 to 5. The piston 22 has a link head 24, which transitions via a neck 42 into a piston body 44. The link head 24 has the spherical surface 26. A through hole 46 extends through the piston 22, which is beveled at one end 48. A total of five axial bore holes 52 having the same diameter D are provided parallel to the through hole 46 and thus to the longitudinal axis L of the piston 22. These extend from the piston base 54 toward the link head 24 and terminate at an axial spacing A to the link head 24. An annular recess 56 for a lid 58 is provided on the piston base 54. The bore holes 52 of the piston 22 form hollow chambers 60, which are disposed on a partial circle, offset to one another. Thus, the hollow chambers 60 are provided in a piston housing 62. As a result of the annular recess 56, a protruding wall section 64 of the through bores 46 remains intact. This wall section 64 forms a coupling part 66 for the lid 58 that is to be coupled thereto, and centers this lid when it is inserted.

As is shown more clearly in FIGS. 6 to 8, the lid 58 is a disk-shaped rotating part having a central bore hole 68 corresponding to the outer diameter of the protruding wall section 64. A spiral groove 72 is provided, as a part of an equalizing mechanism 74, on the outer circumferential surface 70 of the lid 58, which connects the hollow chambers 60 to the environment U in a media conducting manner. When the lid 58 has been placed in the piston housing 62, the spiral groove 72 is covered by the piston housing 62, except for at a discharge point 76. The discharge point 76, as a part of the equalizing mechanism, establishes the media conducting connection between the environment U and the hollow chambers 60 in the piston 22 via the spiral groove 72. The discharge point 76 is disposed on a back surface 80 of the lid 58. The spiral groove 72 opens into an intake point 84 in a bevel 86 at the opposite, front surface 82. The outer diameter of the bevel 86 decreases toward the link head 24, up to a final diameter AD, when the lid 58 is in the inserted state EZ.

As FIG. 4 shows in greater detail, after the lid 58 has been inserted into the piston housing 62, the lid 58 is welded, using an electron beam welding process, along the entire circumference of its internal diameter DI and its external diameter DA. As a result, the discharge point 76 is also closed. The electron beam 88 is oriented parallel to the longitudinal axis L of the piston 22 thereby, in order to enable the most efficient possible welding. For this, the lid 58 should be inserted and oriented in the piston housing 62 without gaps in order to prevent the formation of fissures during the welding. The electron beam welding is carried out in a vacuum, in accordance with DIN 4063. This involves an I-axial seam, in which the electron beam 88 is parallel to the longitudinal axis L. As a result of the welding, the hollow chambers 60 are entirely closed off to the environment U to the outside. After the welding, the piston 22 is finished and heat treated.

The finished piston 22 is depicted in FIG. 5 in a perspective sectional view. It is thus distinguished by a reduced mass, due to the hollow chambers 60, wherein the lid 58 prevents an increase in the dead volume in the cylinder 14. In this manner, no additional compression losses can occur through the hollow chambers 60, as a result of which the efficiency of the hydraulic pump 10 is advantageously improved. 

1. A hydraulic pump, in particular an adjustable axial piston pump, having at least one piston (22) that can move back and forth in a longitudinal direction (18) inside a pump housing (16) when the hydraulic pump is in operation, having a link head (24) and a piston base (54) lying opposite the link head (24), as well as at least one hollow chamber (60), which is at least partially encompassed by a piston housing (60), which closes, substantially or entirely, the respective hollow chamber (60) to the outside.
 2. The hydraulic pump according to claim 1, characterized in that the at least one hollow chamber (60) runs in the direction of, or parallel to, the longitudinal axis (L) of the piston (22) in the piston housing (62), and passes through the piston (22), starting from an axial spacing (A) to the link head (24), to the piston base (54).
 3. The hydraulic pump according to claim 1, characterized in that the at least one hollow chamber (60) is closed by the piston housing (62) or by a lid (58), which can be connected to the piston housing (62).
 4. The hydraulic pump according to claim 3, characterized in that the lid (58) and/or the piston (22) has an equalizing mechanism (74), which connects the at least one hollow chamber (60) to the environment (U) in a media conducting manner.
 5. The hydraulic pump according to claim 3, characterized in that the equalizing mechanism (74) comprises a spiral groove (72) along the outer circumferential surface (70) of the lid (58), which is covered by the piston housing (62) when the lid (58) is inserted into the piston housing (62), except for a discharge point (76), which, as a part of the equalizing mechanism (74), forms the media conducting connection between the environment (U) and the at least one hollow chamber (60) in the piston (22) via the spiral groove (72).
 6. The hydraulic pump according to claim 5, characterized in that the spiral groove (72) opens into a bevel (86) on the lid (58) at its other end, the intake point (84) lying opposite the discharge point (76), or borders on said bevel, wherein the outer diameter of the bevel (86) preferably decreases continuously toward the link head (24), until reaching a final diameter (AD).
 7. The hydraulic pump according to claim 3, characterized in that the piston housing (62) has a coupling part (66) on the piston base (54), by means of which the lid (58) can be coupled to the piston base (54), preferably by welding or brazing.
 8. The hydraulic pump according to claim 3, characterized in that the lid (58) is permanently connected to the piston housing (62) through brazing or an electron beam welding process, and in that the piston (22) is preferably heat treated after the welding.
 9. The hydraulic pump according to claim 8, characterized in that the electron beam welding seam closes at least the discharge point (76) of the spiral groove (72).
 10. The hydraulic pump according to claim 1, characterized in that numerous hollow chambers (60), preferably having the same diameter (D), are provided on a partial circle, offset to one another in the piston housing (62).
 11. A piston for a hydraulic pump according to claim 1, having a link head (24) and a piston base (54) lying opposite the link head (24), as well as at least one hollow chamber (60), which is at least partially encompassed by a piston housing (62), which substantially or entirely closes the respective hollow chamber (60) to the outside.
 12. The piston according to claim 11, characterized in that the at least one hollow chamber (60) runs in the direction of, or parallel to, the longitudinal axis (L) of the piston (22) in the piston housing (62), and passes through the piston (22), starting at an axial spacing (A) to the link head (24), up to the piston base (54).
 13. The piston according to claim 11, characterized in that the at least one hollow chamber (60) is closed by the piston housing (62), or by a lid (58), which can be connected to the piston housing (62). 